1. Field of the Invention
This invention relates to a radiographic intensifying screen and more particularly, to a radiographic intensifying screen comprising a support and a phosphor layer provided thereon which comprises a binder and a phosphor dispersed therein, and optionally a protective film provided on the phosphor layer.
2. Description of Prior Arts
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for non-destructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film for enhancing the radiographic speed of the system. The radiographic intensifying screen consists essentially of a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shocks.
The phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays having passing through an object, the phosphor particles emit light of high luminance (spontaneous emission) in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
It is desired for a radiographic intensifying screen to exhibit a high radiographic speed and to provide an image of high quality (i.e., high sharpness, high graininess, etc.). There has been conventionally proposed a variety of intensifying screens improved in the radiographic speed or the quality of the image provided thereby.
The sharpness of the image in the radiography substantially depends on the spread of the emitted light within the radiographic intensifying screen. The sharpness of the image provided by the intensifying screen is generally enhanced by making the thickness of phosphor layer smaller, but in this case the radiographic speed thereof is apt to decrease. Therefore, for attaining the enhancement of sharpness of the image without decreasing the radiographic speed, it is desired that the mixing ratio between the binder and the phosphor (binder/phosphor) in the phosphor layer is made smaller so as to give a phosphor layer containing the phosphor in a large amount.
The radiographic intensifying screen hardly deteriorates upon exposure to a radiation, the intensifying screen can be repeatedly employed for a long period of time. Accordingly the intensifying screen is also required to have a sufficient mechanical strength so as not to allow easy separation of the phosphor layer from the support (and from the protective film in the case that the protective film is provided on the phosphor layer), when mechanical shocks and mechanical force caused by falling or bending are applied to the screen in the use.
However, the radiographic intensifying screen has a tendency that the bonding strength between the phosphor layer and support as well as that between the phosphor layer and protective film decreases as the mixing ratio of the binder to the phosphor in the phosphor layer decreases, in other words, as the amount of the phosphor contained therein increases.
For instance, it has been heretofore proposed to employ cellulose derivatives as a binder of phosphor layer of the radiographic intensifying screen from the viewpoint of dispersibility of the phosphor in the binder solution (i.e., coating solution), but in this case the obtained intensifying screen has not the mechanical strength enough for preventing easy separation of the phosphor layer from the support. It has been also proposed to employ a polyester resin as the binder of the phosphor layer from the viewpoint of resistance to flexing and adhesion thereof to the support, but in this case it is difficult to obtain a phosphor layer containing a phosphor in a large amount.
Further, in the case that a phosphor layer is formed on a support by a conventional coating procedure using the above-mentioned binders, the phosphor particles are apt to separate from the binder in the drying procedure of the phosphor layer, because the binders have poor affinity for the phosphor. As a result, the relatively large amount of phosphor particles gather on the support side of the phosphor layer, and accordingly, the phosphor particles are present in a relatively small amount on the screen surface side of the phosphor layer (or the protective film side, that is, the side which is in contact with a radiographic film) so as to produce so-called "gathering on surface" of binder. In such radiographic intensifying screen, especially when the phosphor layer contains the phosphor in a large amount, the phosphor particles aggregate on the support side of the phosphor layer, whereby the enough bonding strength between the phosphor layer and support cannot be obtained. In addition, the light emitted by the phosphor easily spread on the screen surface side of the phosphor layer because of the gathering on surface of the binder, so that the quality of the image tends to deteriorate.
On the other hand, in order to enhance the bonding strength between the phosphor layer and protective film in the radiographic intensifying screen comprising a support, phosphor layer and protective film, it has been proposed to employ the known acrylic resin such as a polyalkyl methacrylate as the binder of the phosphor layer, but there is a tendency that the cracks are produced in the phosphor layer when the mechanical shock such as bending is given to the screen.